ch 9 noncombustible construction
TRANSCRIPT
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9 Noncombustible
Construction
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Objectives (1 of 2)
• Understand the difference between noncombustible and fire-resistive construction
• Identify the different types of steel building components and their characteristics
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Objectives (2 of 2)
• Describe different types of steel structural systems
• Describe the hazards of a metal deck roof fire
• Understand the hazards of high fire loads in unprotected steel structures and ways to improve the situation
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Introduction• Noncombustible and fire resistive
construction
• Differ in the level of fire resistance assigned to the structural frame, walls, floors, and roof
• Noncombustible construction has little fire resistance
• Fire resistive construction has moderate to heavy fire resistance
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Noncombustible Construction
• Allowable area and height is much less than fire resistive construction
• Maximum height is 12 stories
• Fire resistive can have unlimited height
• Fire-resistive construction can use steel for its framing system
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Steel
• Modulus of elasticity about 29 million pounds per square inch (psi)
• High tensile strength and shear strength
• Strong but lightweight members have little inherent fire resistance
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Fire Characteristics of Steel
• Substantial elongation
• Above 1300°F, steel members may fail
• Cold-drawn steel will fail at about 800°F
• Steel transmits heat readily
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Unwarranted Assumptions
• False belief in steel’s “fireproofness”
• Need to set priorities
• Heat absorbed by contents or structural elements is the most important heat
• Heat being evolved from contents that are burning is of secondary importance
• Heat leaving the structure—let it go
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Water on Hot Steel
• Water is the fire department’s heat removal medium
• Myth: Water should not be thrown on heated steel
• Cooling effect of water draws steel back to its original dimensions
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Definitions: Steel Construction Members (1 of 3)
• Angles
• Bars
• Box columns
• Box girders
• Channels
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Definitions: Steel Construction Members (2 of 3)
• I-beams
• Plates
• Purlins
• Rolled or built-up members
• Spandrel girders
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Definitions: Steel Construction Members (3 of 3)
• Tees
• Tubes
• Weight
• Wide-flange shapes
• Zees
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Steel as a Construction Material (1 of 2)
• Makes it possible to erect tall buildings
• Has consistency in structural characteristics
• Can be connected to other structural elements
• Used for fire escapes
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Steel as a Construction Material (2 of 2)
• Provides the tensile strength that concrete lacks
• Used in concrete flooring systems
• Used to repair failures in concrete buildings
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Steel Buildings
• Used in peaked roofs
• Bar joists span the main trusses to support a flat roof
• Steel is almost universally unprotected.
• Buildings often can only be classified as noncombustible
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Protected Noncombustible Sprinklered Construction
• Found occasionally
• Major structural elements have fire resistance
• Building itself is not fire resistive
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Rigid Frames
• Column is narrow at the base and tapers to its widest point at the top
• Girder is also tapered
• Wide haunch resists the outward thrust of the roof
• Clear spans of about 100 feet
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Steel-Framed Buildings
• Many are prefabricated
• Butler Company is a prominent manufacturer
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Huge Spans
• Span collapses can be sudden and tragic
• Adjacent bents are tied together
• Tying the steel units together creates dependencies between torsional or eccentric loads
• The higher the resistance to wind load, the more likely a progressive collapse
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Developing Wide-Span Trusses
• Designs may push the limits of steel
• Hasty field changes or errors in construction can have catastrophic consequences
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Deep Parallel-Chord Trusses
• Floor beams in hospitals
• Interstitial space
• Such voids should not be used for storage or maintenance
• Automatic sprinklers should be required
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Heavy Parallel-Chord Trusses
• Have been used as transfer beams
• Often hidden in partition walls
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Trussed Arches
• Arch of a steel arch bridge is often a truss
• Is a compression structure
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Walls of Steel-Framed Buildings
• Wall composition varies
• Metals, cement-asbestos board, masonry, concrete, and reinforced plastics found
• Wall insulation and coatings also factors
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Cement-Asbestos Board
• Noncombustible and is often used for friable construction
• Friable construction is used where an explosion is a possibility
• Will break away readily and relieve pressure
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Glass-Fiber Reinforced Plastics
• Noncombustible
• Resinous binder most often used with it is flammable
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Aluminum
• Noncombustible, but has a low melting point
• Has little mass per unit of area, so it disintegrates rapidly in a fire
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Precast Prestressed Concrete Panels
• Usually erected in large sections
• Collapse is hazardous to fire fighters
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Masonry Walls
• Often used for walls for unprotected steel-framed buildings
• Made of concrete block or a composite
• Usually only curtain walls
• Important to analyze the effect of the expansion of the steel frame on the wall
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Galvanized Steel Walls
• Used when heat conservation is not important
• Asphalt asbestos protected metal (AAPM)
• Robertson Protected Metal (RPM) is one proprietary name
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Metal Panels
• Prefabricated metal panels in a sandwich construction
• Plastics are often used with metal panels
• Insulation, vapor seal, or adhesive in the panels may be combustible
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Polyurethane Insulating Panels
• Protected by gypsum board and stainless-steel sheathing
• If a cutting torch is later used, a smoky, destructive fire may result
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Aluminum Sandwich Panels
• Can be made with foamed polyurethane
• Some are listed by Underwriters Laboratories (UL) Inc. for low flame spread ratings
• Smoke-developed ratings may be quite high
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Failure of the Closure of the Wall Panel to the Floor Slab
• Design of panel walls
• Method of installation
• Degradation of insulation
• Expansion of metal under fire conditions
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High-Rise Framing
• Steel once stood unchallenged as a method for high-rise buildings
• Concrete now is finding more use
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Builders’ Hesitation
• Brick, stone, and terra cotta added to framed buildings
• Goal was to reduce the apparent or perceived height of the building
• Didn’t openly discuss use of steel-frames
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Tilt-Slab Hazards
• Walls braced with tormentors or braces until the roof secured
• If the roof is being lost in the fire, beware of wall collapse
• If heavy smoke is present, the sprinklers are not controlling the fire
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Steel-Framed Buildings Under Construction
• Wind forces must be resisted, because the building is not fully connected
• Braces may not be properly installed
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Plastic Design in Steel Construction
• Connections are built to transfer loads beyond the column
• Beams are lighter and columns are smaller than they would be otherwise
• The lighter the steel, the less fire resistance
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More on the Fire Characteristics of Steel
• Conducts heat
• Elongates as temperature increases
• Loses strength at high temperatures
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Steel Conducts Heat
• Steel transmits heat
• Tin ceilings can transmit fire
• The conductivity of steel can be a factor in spreading fires
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Ships
• Practice of using ships as buildings is growing
• Ships have steel walls known as bulkheads
• Welding operations are performed without concern for heat transmission
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Self-Storage Facilities
• Have many of the characteristics of ships
• Fire can spread from unit to unit by conduction and radiation
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Steel Elongates
• Expands from 0.06 percent to 0.07 percent for each 100°F rise
• At 1000°F, a steel member will expand 9 1/2 inches over 100 feet of length
• Above 1000°F, steel starts to soften and fail
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Elongating Steel
• Exerts a lateral force against the structure that restrains it
• Expansion of steel may cause the displacement of masonry
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Hot, Fast Fires Effect on Steel Buildings
• Failure temperatures are reached rapidly
• Lateral thrust against the wall is minimized
• Overturning can be anticipated
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Steel Fails• Steel above 1000°F starts to lose strength
rapidly
• National Fire Protection Association (NFPA) 251 (American Society of Testing and Materials (ASTM) E119) test reaches 1000°F in five minutes
• National Institute of Standards and Technology (NIST) test reaches 1500°F in five minutes
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Standard Tests of the Fireproofing of Steel Columns
• Test ends when a temperature of 1200°F is exceeded at one point or 1000°F is exceeded on the average in the column
• A principle variable is the weight or mass of the steel unit
• Ventilation is also a factor
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Overcoming the Negative Fire (1 of 2)
• Ignore the problem
• Rely on an inadequate code
• Take a calculated risk
• Fireproof (insulate) the steel
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Overcoming the Negative Fire (2 of 2)
• Protect the steel with sprinklers
• Fireproof the steel with a water cooling system
• Locate the steel out of range of the fire
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Ignoring the Problem
• Potential for fire damage to steel buildings is not clearly understood
• Unwarranted confidence on the fact that the steel is noncombustible
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Steel Highway Structures and Bridges
• Unprotected steel that is vulnerable to an occasional gasoline truck fire
• New York City Fire Department has had preplans in place for the East River bridges for over 70 years
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Hazards of Concentrated Fire Loads (1 of 2)
• Unprotected steel buildings may have highly concentrated fire loads
• “One-high story” buildings with internal structures
• Mezzanines, sometimes built of wood
• Other combustible spaces
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Hazards of Concentrated Fire Loads (2 of 2)
• Metal trailers are hazardous, especially when grouped
• Prefabricated buildings, especially with certain types of insulation in walls
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Excavation Bracing
• Building excavations are being made much deeper
• Walers and rakers
• An excavation is loaded with combustibles
• Tiebacks
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Buildings Under Construction
• Steel may be unprotected for extended periods
• World Trade Center used ordinary plywood to save $1 million
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Relying on Inadequate Codes
• Building Codes
• Unprotected noncombustible or protected noncombustible
• “Protection” refers to physical protection of the steel with gypsum board, spray-on fireproofing, or the like
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Study the Type of Construction
• Column, girder, and beam construction is common
• If a masonry bearing wall is substituted for some of the exterior columns, the building is wall-bearing
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Assumption About Fires
• They burn only upwards
• Example: A fire involving a wooden balcony or a metal deck roof could well cause steel framing to move and thus cause brick-veneered walls to fall
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Steel High Above the Floor
• Codes vary in terms of protection of steel buildings
• Heights 20-30 feet above the floor are often left unprotected as risks are thought to be low
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The McCormick Place Fire
• The main exhibit area provided a clear area of 320,000 square feet
• The columns were trusses themselves.
• The columns were fire protected up to a height of 20 feet
• The roof trusses were unprotected
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What Could Have Been Done Better
• Modern building codes would have required automatic sprinkler system
• Sprinklers could have limited the spread of this fire
• Codes were inadequate
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The New McCormick Place
• Structural steel is protected with directly applied fireproofing delivering one-hour fire resistance
• The entire building is sprinklered
• Provisions have been made for smoke venting
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Important Test Experiences
• Underwriters Laboratories tested in the aftermath of the fire
• Tests done in building 30 feet high with typical fuels
• The first test fires had 1500°F after 5 minutes and 45 seconds
• A bar joist reached 1540°F and an I-beam 1355°F in just over 5 minutes
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Taking Calculated Risks
• Financial Calculation
• Engineering Calculation
• “Forgetting It” Calculation
• Steel Industry
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Insulated Metal Deck Roof Fire Problems
• Fire in the General Motors transmission plant at Livonia, Michigan
• The metal deck roof was the principal contributing factor to the destruction of the plant.
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Insulation
• Is useless when it absorbs moisture
• Must be protected from capillary attraction
• A bituminous coating serves as the adhesive and sometimes as a moisture-stopping vapor barrier
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An Approved Roof
• Is one that meets UL standards
• Can be UL listed but still be a combustible metal deck roof
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When a Fire Occurs
• The metal deck heats up
• Heat is conducted through the deck to the bituminous adhesive
• Adhesive liquefies and then vaporizes.
• When the gas mixes with the air below, it ignites from the fire below
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Prevention of Metal Deck Roof Fires
• Use Factory Mutual Class I roofing or a UL Classified Roof
• Provide adequate automatic sprinkler protection for the roof, even though the contents may be noncombustible
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Fires of Interest (1 of 2)
• Tinker Air Force Base
• A fire was started by roofers
• The sprinklers were below a wire lath and plaster ceiling
• The water did not hit the underside of the roof deck, and the fire burned unimpeded
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Fires of Interest (2 of 2)
• Wabush Mines
• A fire in a large building damaged a metal deck roof 90 feet above the floor
• Even if the roof had been Class I, damage would have been severe
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Class I
• Some think it means “completely satisfactory under all circumstances”
• A Class I roof with any combustibles should not be used over a high-value installation
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Atomic Energy Commission (AEC)
• Studied fire protection for thousands of acres of metal deck roofs
• Decided to add sprinklers to government plants
• Over $20 million was spent on sprinklers and water supplies
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An Unrecognized Problem (1 of 2)
• Metal deck roof a factor in many losses
• Common characteristics of metal-deck roof fires
• Small openings allow rapid fire spread
• Thick, black, choking smoke, sometimes with dripping tar
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An Unrecognized Problem (2 of 2)
• Fire suppression group may not understand the significance of these fires; building codes often are inadequate
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Fighting the Metal Deck Roof Fire
• Marine Corps Supply Depot in Norfolk, Virginia
• Marine plywood office
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Metal Decks on Nonmetal Buildings
• Metal decks found on steel-framed buildings as well as masonry buildings
• Kensington, MD Fire
• In December 1970, a fire broke out in a janitor supply business
• Chief identified as probable metal-deck roof fire
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Types of Protection of Steel Structures
• Unprotected
• Dynamic protection
• Passive protection
• Passive/dynamic combination
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Unprotected Steel
• Has potential for early collapse
• Wichita, Kansas automobile showroom fire
• Repair bays built of lightweight steel truss construction
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Need to Cool All Heated Steel
• The quantity of water is not excessive
• Cool all the steel that is within reach of hose streams and give special attention to columns
• Solid stream tip might be better than a fog tip
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Water Damage
• Some argue against the use of water because of the damage it can cause
• If owners had sprinklered the building, water would be discharged anyway
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Dynamic Fire Protection
• Accomplished with various types of automatic sprinkler systems
• Hydraulically calculated sprinkler design is no guarantee of success
• Heavy structural steel sometimes is protected by special lines of sprinklers.
• Deluge and fog/foam systems used for flammable liquids
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Passive Fire Protection (1 of 2)
• Is the legally required level of fire resistance adequate for the fire load as it exists in the building?
• Has the protection of steel been provided, and is it maintained?
• Is there any legal relief?
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Passive Fire Protection (2 of 2)
• Is it up to the fire department simply to do the best it can in the event of a fire?
• If the fire department estimate of the situation indicates potential or inevitable disaster, who, if anyone, is notified?
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What All Personnel Should Know
• The requirements for fire resistance as applied to specific buildings
• The manner in which fire protection can be degraded is also crucial
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Degradable Methods
• Sprayed-on protection
• Membrane fireproofing
• Fire-rated tile
• Considerations
• Permitted under local code?
• Could tampering have occurred which might make less effective?
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Passive/Dynamic Protection
• Partial static protection (such as a spray-on coating) used along with automatic sprinklers
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Code Problems• Develop competence
• One person or group should become familiar with current and past local building codes
• Building officials may be less than enthusiastic about fire fighter involvement.
• Some fire prevention managers are equally unenthusiastic about fire fighter involvement
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Code Variances
• Code exceptions might be made by building and fire prevention officials
• Codes do not permit waivers to specific requirements
• All modifications to requirements of the code should be fully documented
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Preplanning Your “McCormick Place”
• Make your own case study
• Consider a possible preplan of a fire in an unprotected steel building being used as an exhibit hall
• Assume there is a balcony running down both sides that is used for sports events but not for exhibits
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First Moves in a Case Study
• Battle for sprinkler protection or denial of the facility to high fire-load exhibits
• After you lose, get in writing an agreement that the fire department can take whatever steps necessary to protect life and property during exhibits
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Consider a Worst Case Scenario (1 of 2)
• An exhibition with a potential for a high rate of heat release in a fire
• A hot, fast fire is anticipated indicates a more severe test of the structure than a slower fire
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Consider a Worst Case Scenario (2 of 2)
• Do you need an alternative automatic system?
• Study the potential water supply and needs
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Be Proactive
• Watch for new ideas
• Step-by-step logic leads to the practical solution
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Summary (1 of 2)
• Steel is the most important metal used in building construction
• Steel has several important characteristics to consider regarding its behavior in fire.
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Summary (2 of 2)
• Steel structures can be divided into the following types:
• Unprotected
• Dynamically protected
• Passively protected
• Passive/dynamic combination protection
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